The visible light emission from Tm3+-doped glass is remarkable because it involves a process in which multiple photons of lower energy are absorbed by the material and then re-emitted as a photon of higher energy. This process is known as upconversion and may occur by several different paths such as sequential upconversion and cooperative upconversion.
In sequential upconversion, a single ion (for example, Tm3+) absorbs a photon via excitation of an electron from the ground state to a metastable excited state. Before the excited electron returns to the ground state, the same ion absorbs a second photon, and the electron is further excited to a higher energy state. This second excitation is known as excited state absorption (ESA) and can occur several times, with the electron moving to successively higher energy levels. When the excited electron undergoes radiative decay, it can emit a photon having more energy than the excitation photon. The exact energy of the emitted photon will depend on the energy levels of the fluorescent ion. In sequential upconversion, a single ion must absorb two or more photons, so the efficiency of the upconversion process is highly dependent on the lifetime of the metastable intermediate state.
Cooperative upconversion involves energy transfer between two or more excited ions. If the ions are close enough, one excited ion can transfer energy to a second excited ion. The electron on the first ion moves to a lower energy level while the electron on the second ion moves to a higher energy level. The ions exchanging energy may be of the same type or may be of different types. Because energy must transfer between ions, this process is highly dependent on the concentration and distribution of excitable ions in the glass host as well as the lifetime of the metastable state.
Upconversion fluorescence is known in Tm3+-doped silicate and Tm3+-doped fluoride fibers. To date, most of the research has been done on fluoride fibers because the lower phonon energy of the host fluoride glass reduces the non-radiative decay rate of the excited states of the Tm3+ ion. With decreased non-radiative decay, the intermediate metastable states have longer lifetimes, and therefore the fluoride fibers are believed to be more efficient. The fluoride glasses are good hosts for Tm3+, and Tm3+-doped infrared-pumped visible fluoride fiber lasers have been demonstrated. However, the fluoride fibers have the disadvantage that they are much more difficult to manufacture and handle, and therefore much more expensive than silicate-based fibers.
In silicate hosts, Tm3+ has been doped into SiO2—GeO2 and SiO2—Al2O3 fibers. Hanna, D. C. et al., Optics Communications, 78 (1990) 187; Lincoln, J. R. et al., Journal of Luminescence, 50 (1991) 297; Tanabe, S., E. Snitzer, and B. Cole, Japanese Journal of Applied Physics, 37 Suppl. 37-1 (1998) 81. Tm3+ upconversion has also been studied in silica-on-silicon waveguides (SiO2—P2O5) and in bulk sol-gel silica glasses (SiO2—Al2O3) Bonar, J. R. et al., Optics Communications, 141 (1997) 137; Vermelho, M. V. D. et al., Optical Materials, 17 (2001) 419; Otto, A. P., K. S. Brewer, A. J. Silversmith, Journal of Non-Crystalline Solids, 265 (2000) 176. In some studies, Yb3+ was co-doped into the system as a sensitizer for the Tm3+ upconversion. Work by Lincoln et al. showed that the lifetime of the 1G4 and 3F4 excited states was longer in SiO2—Al2O3 than in SiO2—GeO2 fiber. Lincoln, J. R. et al., Journal of Luminescence, 50 (1991) 297. However, studies in a silicate fiber found, “A problem with a silica host is that significant non-radiative decay occurs from the 3F4 level [the 3H4 level in the notation used in this document] (a consequence of the small energy gap between this level and the 3H5 level and the high phonon energy of silica) which means that the lifetime of this level is significantly shorter than that observed in, for example, a fluorozirconate host with a corresponding reduction in the upconversion efficiency.” Hanna, D. C. et al., Optics Communications, 78 (1990) 187.